Thermoplastic resins have a lower specific gravity than glass or metal and excellent properties such as moldability and impact resistance. Plastic resins have been widely used in various products such as automobiles and electrical/electronic appliances as a substitute for glass or metal to lower costs, scale up production, reduce weight, and the like. For this reason, the function and appearance of thermoplastic resins used in housings are increasingly important, and scratch resistance and flame resistance are also increasingly required.
Polycarbonate (PC) resin can have excellent impact resistance, thermal stability, self-extinguishing properties and dimensional stability as well as excellent mechanical strength and flame resistance. Therefore polycarbonate resin is used to manufacture various products such as automobiles and electrical/electronic appliances, among others. Moreover, glass in products that require both impact resistance and transparency such as lens can be substituted with polycarbonate resin. However, polycarbonate has low scratch resistance.
Acrylic resins, especially polymethyl methacrylate (PMMA) resin, can have excellent transparency, weather resistance, mechanic strength, surface gloss, adhesion, and scratch resistance. Acrylic resins, however, can have poor impact resistance and flame resistance.
One method to improve scratch resistance of a resin surface includes hardening an inorganic or organic hybrid material on the surface of a final molded resin using heat or ultraviolet light. However, this method can increase costs, cause pollution problems and increase processing times because of the additional coating step. Accordingly, there is increasingly a need for a non-coated resin exhibiting scratch resistance. Moreover, development of a resin with excellent scratch resistance is very important for the housing industry.
One method for preparing a resin having both scratch resistance and flame resistance that also addresses the above problems is preparing a PC/PMMA resin by mixing polycarbonate and acrylic resin, preferably polymethyl methacrylate (PMMA). However, the PC/PMMA alloy is difficult to color, has poor transparency and forms a distinct melt joint during injection because the PC/PMMA alloy diffuses light and has high chroma due to the difference between the refractive indices of PC and PMMA. Moreover, the composition can exhibit phase separation during melt mixing at a high temperature due to low compatibility between PC and PMMA.
KR Patent No. 2004-79118 discloses reducing the molecular weight of polycarbonate during mixing using metal stearic acid ester to improve the compatibility between PC and PMMA. However, the blend of polycarbonate and methacrylate resin has limited transparency.
Generally, when the molecular weight of a resin is reduced in an alloy of resins with low compatibility, compatibility tends to increase. Therefore, if the molecular weight of PMMA is low, compatibility with PC tends to improve, but transparency is not expected due to phase separation and haze on the surface due to the difference between the refractive indices of the PMMA and PC.